Idk in others platforms but on android i just fix (bugzilla post) that by installing version 66.02 and change value in about:config for:
"xpinstall.signatures.required false".
In Android I would suggest using Fennec (which can be downloaded using F-Droid). On my android phone this uh, bug? does not seem to affect anything and Mozilla's telemetry is disabled by default. Granted, you can disable it manually in any firefox browser by typing telemetry in the search box in about:config and setting the values you do not feel comfortable with there to false.